There are many ways to calculate quantiles (from Wolfram MathWorld). The default method used by @quantile will always return NA for the 0-th quantile, regardless of whether there are NAs in your data or not. I believe the only quantile method that would return the minimum of the series for the 0-th quantile is the Gumbel method (add option "g" to @quantile). I assume you want quantiles other than the 0-th otherwise you could just use @min.